Production of alkyl silicates

ABSTRACT

This invention involves the production of an alkyl silicate in which the alkyl group has from one to three carbon atoms involving the passage of an electric current through a liquid medium comprising a solution of a monohydric alcohol having from one to three carbon atoms per molecule and as a supporting electrolyte an acid or metal salt compatible with the monohydric alcohol with no more than 3 percent by weight of water based on the weight of the monohydric alcohol being present using a silicon anode and passing the electric current until alkyl silicates are present in the liquid medium.

Jan. 9, 1973 PRODUCTION OF ALKYL SILICATES raven (iii Taste; 713F565 -Tripp,Chester, v

England Assignee: Monsanto Chemicals Limited, London,England [22] Filed: Feb. 10, 1970 [21] Appl. No.: 10,300

[30] Foreign Application Priority Data Feb. H, 1969 Great Britain ..7,342/69 [52] Cl ..204/59 [51] Int. Cl. ..B0lk 3/00 [5 8] Field of Search ..204/59 [56] References Cited FOREIGN PATENTS 0R APPLICATIONS 1,136,0l6 12/1968 Great Britain ..204/59 Primary Examiner-Howard S. Williams Attorney-Herbert B. Roberts, Roger R. Jones and Neal E. Willis [57] ABSTRACT This invention involves the production of an alkyl silicate in which the alkyl group has from one to three carbon atoms involving the passage of an electric current through a liquid medium comprising a solution of a monohydric alcohol having from one to three carbon atoms per molecule and as a supporting electrolyte an acid or metal salt compatible with the monohydric alcohol with no more than 3 percent by weight of water based on the weight of the monohydric alcohol being present using a silicon anode and passing the electric current until alkyl silicates are present in the liquid medium.

11 Claims, No Drawings PRODUCTION OF ALKYL SILICATES This invention relates to a process for the production of alkyl silicates.

Alkyl silicates are well known chemical compounds with established commercial uses, for example as binding agents in the production of refractory moulds for metal casting. The usual commercial method for their production comprises the reaction of alcohols with silicon tetrachloride, which is itself obtained by the action of chlorine on silicon. This method suffers from the economic disadvantages of any multistage process, and a method for the direct conversion of silicon to alkyl silicates is therefore desirable. Such a method is provided by the present invention.

The method is related to that of our British Pat. Specification No. 1,136,016 which describes and claims a process for the production of a silica so], in which an electric current is passed through a liquid medium comprising water and a monohydric alcohol having from one to three carbon atoms in the molecule and containing sulphuric acid or hydrochloric acid as a supporting electrolyte, using an anode comprising silicon in contact with the liquid medium. In our British Pat. Specification No. 3641/69 we disclose that certain metal salts can function as supporting electrolytes in an electrolytic process of the above type.

We have now found that alkyl silicates are also formed in this process, probably as the primary products of the anodic reaction, and it is thought that the colloidal silica arises as a result of the hydrolysis of the alkyl silicates by the water present in the liquid medium.

It follows that if the amount of water initially present in the liquid medium is kept to a minimum and the electrolysis is continued until this water has been consumed, the alkyl silicates formed thereafter will persist as such in the liquid medium, from which they can be isolated at the end of the electrolysis. Under suitable .conditions, the alkyl silicates obtainable in this way represent a major proportion of the silicon of the anode consumed during the electrolysis.

The process of the present invention is accordingly one for the production of an alkyl silicate, in which the alkyl group has from one to three carbon atoms, in which an electric current is passed through a liquid medium comprising a solution, in a monohydric alcohol having from one to three carbon atoms per molecule, of an acid or a metal salt that is compatible with the monohydric alcohol and is ionized to a sufficient extent and is present in sufficient amount to function as a supporting electrolyte, the said medium containing not more than 3 percent of water based on the weight of the monohydric alcohol, using an anode comprising silicon in contact with the liquid medium, and passage of the electric current is continued until alkyl silicate persists in the liquid medium and for a period thereafter.

Preferably the water content of the liquid medium is less than 1 percent by weight for example from to 0.5 percent by weight and preferably the electrolysis is continued until the liquid medium contains at least 20 percent by weight of silicon compounds in terms of silica, and even more preferably until the liquid medium contains from 25 percent to 30 percent by weight of silicon compounds in terms of silica.

The monohydric alcohols having from one to three carbon atoms in the molecule, useful in the process of the invention include the alkanols, methanol, ethanol, prppanol and isopropanol and mixtures of these alcohols. Ethanol is the preferred alcohol.

The acids useful in the process include sulphuric and hydrochloric acids, the former being generally preferred because of its low volatility. The acid is essentially only a current carrier and the amount employed is therefore usually the minimum required to give a practical conductivity. This minimum obviously depends on a number of factors but may typically be about 1 percent by weight of the liquid medium. It is preferable that the concentration of the acid in the liquid medium should not exceed 5 percent by weight and should not be less than 0. 1 percent by weight.

Suitable metal salts include metal chlorides and metal sulphates, for example zinc chloride, cupric sulphate, nickel sulphate, magnesium sulphate, zinc sulphate and ferric sulphate, of which the latter is the preferred salt for use in the present process. Hydrated or anhydrous salts can be used, but in many instances the former have higher solubilities in the lower monohydric alcohols and are therefore more suitable for obtaining liquid media having the required conductivities. The metal salt must have sufficient solubility and ionisation in the liquid medium to produce a solution which has sufficient conductivity to act as a current carrier. The amount of such a metal salt which needs to be dissolved in the liquid medium to give a practical conductivity obviously depends on a number of factors. However, a practical lower limit for the specific conductivity of the liquid is about 2.4 X 10 'ohm cm". At lower conductivities the process is unduly prolonged. Preferably the liquid medium has a specific conductivity of at least 3.0 X 10, for example a conductivity within the range 10'' to 10' ohm cm such as for instance from 3 X 10" to 5 X 10 ohm cm".

In the case where the metal salt is ferric sulphate nonahydrate, satisfactory results are obtained using absolute ethanolic solutions having concentrations of from 4 to 8 grams of the salt per liter.

The liquid medium is preferably free from components other than those specified above, but there may be present an inert organic liquid miscible with the liquid medium e.g. a ketone, an ether or an ester a liquid diluent if required. For example the presence of up to 25 percent of acetone based on the volume of the liquid medium can be tolerated. Where such a diluent is present, the basis for the water content of the medium as discussed above, is the combined weight of the monohydric alcohol and diluent.

The anode employed may consist of substantially pure silicon, and various semi-conductor grades of the material, i.e. silicon in which essentially the only impurity is a substance added in controlled amount to impart conductivity of a determined type and magnitude, can be used successfully. It is, however, impractical to use for the anode silicon having a resistivity significantly more than 10ohm-cm. in the present process; silicon having a resistivity in the range 0.005 to 1.0 ohmcm. is generally the most satisfactory.

For economic reasons, the anode employed in the process of the present invention is usually fabricated from a compound or alloy of silicon rather than from a semi-conductor grade of the element. Ferrosilicons have been found to be especially suitable. The silicon content of the ferrosilicon can, for example, be within the range 60 to 99 percent or 70 to 77 percent by weight, and in typical instances maybe within the ranges 70 80 percent, 90 95 percent or 97 99 percent by weight.

Any inert conductive material can serve as the cathode. Graphite is a useful cathodic material having a considerable advantage in terms of cost over other functionally suitable materials such as platinum or silver.

The electric current used in the process of the invention can be a simple direct current or rectified A.C. with or without smoothing. The current density at the anode may typically be within the range of from 7 to 200 milliamps per square centimeter, although operation at current densities over a wider range than this is possible. For economic reasons, the voltage applied in excess of the decomposition voltage of the system should be kept as low as possible. In practice, the operating voltage is determined by factors such as cell design and electrolyte concentration, and wide variations are possible.

The electrolysis can be conducted at a range of temperatures. The generally most convenient manner of operating is to provide the electrolysis cell with a reflux condenser and to carry out the process at the boiling point of the liquid medium. Often the resistive heating by the current is sufficient to maintain the liquid medium at its boiling point. The process can be conducted at lower temperatures, however, for example from 10C. up to the boiling point of the liquid medium, but it is then generally necessary to provide cooling means.

lt is usual to carry out the electrolysis in the presence of an inert atmosphere e.g. nitrogen, but this is not essential.

The precise nature of the product of the electrolysis depends on the process conditions, in particular the water content of the liquid medium, but it is usually a mixture comprising alkyl orthosilicates and alkyl polysilicates of varying degrees of condensation. Some colloidal silica may also be formed if the initial water content of the liquid medium is relatively high. The components of the mixture can be separated or partially separated if desired by conventional techniques, for example, fractional distillation under reduced pressure, generally after neutralising any acid present, column chromatography or gas chromatography. Where the product is required as a binding agent in the production of refractory articles, however, such separation is unnecessary.

The invention is illustrated by the following Example.

EXAMPLE A mixture of concentrated sulphuric acid (5 ml.) and absolute ethanol (300 ml.) was electrolysed under an atmosphere of nitrogen in a flask fitted with a graphite cathode, a ferrosilicon anode and a reflux condenser. A direct current of 4 amps. was passed through the cell although the strength of the current diminished towards the end of the run. The initial applied voltage was 33 34 volts which was later increased to 60 70 volts. A total of amp. hours was supplied to the cell during the run. Resistive heating heated the electrolyte to its boiling point and kept it'under reflux throughout the remainder of the run.

The electrolyte was made neutral to bromophenol blue by the addition of a solution of sodium ethoxide in ethanol. Ethanol was then distilled off to leave a liquid having more than half the volume of the original electrolyte. This liquid was distilled under reduced pressure to yield two fractions of hp ll0 122 C./9 mm. Hg and b.p. 250 C./8 mm. Hg. and a residue which did not distill at 300 C./8 mm. Hg. The first and second fraction had a silicon compound content of 28.9 percent and 47 percent by weight in terms of silica respectively. The nuclear magnetic resonance spectra of these fractions showed the presence of very little tetraethyl orthosilicate but. showed the presence of mainly straight chain and branch chain polysilicates. The specific gravity of the first and second fractions gave values for the ratio of silicon atoms to ethoxy groups of 2.4 and 1.9 respectively using the method described in J. Inorg Nuclear Chem., 1967, 29, 78 which agreed closely with the ratios deduced from the n.m.r. spectra of the fractions.

The electrolytic process gave a product with a total content of silicon compounds expressed as silica of 27.6 percent by weight and thus a current yield expressed as silica of 0.49 gm. of silica per amp. hour. The product of the electrolytic process consisted of 35 percent by weight of ethanol, 25 percent by weight of material boiling between ll0C,/9 mm. Hg. and 250 C./8 mm. Hg. and 40 percent by weight of undistilled material.

What is claimed is:

l. A process for the production of an alkyl silicate in which the alkyl group has from one to three carbon atoms comprising passing an electric current through a liquid medium comprising a solution of an acid or a metal salt in a monohydric alcohol having one to three carbon atoms per molecule, said medium containing not more than 3 percent by weight, based on the weight of said alcohol, of water initially until any hydrous medium is converted into an anhydrous medium and thereafter until said alkyl silicate persists in said anhydrous liquid medium using an anode comprising silicon in contact with the liquid medium, said acid or metal salt being compatible with said monohydric alcohol, being ionized to a sufficient extent, and being present in sufiicient amount to function as a supporting electrolyte.

2. A process according to claim 1 wherein said liquid medium contains not more than 1 percent by weight of water..

3. A process according to claim 2 wherein said liquid medium contains from about 0 to about 0.5 weight percent of water.

4. A process according to claim 3 wherein the supporting electrolyte is selected from the group consisting of sulfuric acid, hydrochloric acid, a metal chloride, and a metal sulfate.

5. A process according to claim 4 wherein said supporting electrolyte is selected from the group consisting of hydrochloric acid, sulfuric acid, ferric sulfate and I zinc chloride.

6. A process according to claim 5 wherein said liquid medium is anhydrous.-

is a ferro-silicon containing from 60 to 99 percent by weight of silicon.

10. A process according to claim 9 where the process is conducted at substantially the boiling point of the liquid medium.

11. A process according to claim 9 in which the liquid medium is a solution having a specific conductivity of at least 3.0 X 10' ohm", cm to about 3.0 X 10' ohm", cm" at the operating temperature. 

2. A process according to claim 1 wherein said liquid medium contains not more than 1 percent by weight of water.
 3. A process according to claim 2 wherein said liquid medium contains from about 0 to about 0.5 weight percent of water.
 4. A process according to claim 3 wherein the supporting electrolyte is selected from the group consisting of sulfuric acid, hydrochloric acid, a metal chloride, and a metal sulfate.
 5. A process according to claim 4 wherein said supporting electrolyte is selected from the group consisting of hydrochloric acid, sulfuric acid, ferric sulfate and zinc chloride.
 6. A process according to claim 5 wherein said liquid medium is anhydrous.
 7. A process according to claim 5 wherein the supporting electrolyte is sulfuric acid or hydrochloric acid and is present in said liquid medium from about 0.1 to about 5 percent by weight, based on the weight of the liquid medium.
 8. A process according to claim 7 in which the passage of the electric current is continued until the liquid medium contains from about 25 percent to about 30 percent by weight of the silicon compounds in terms of silica.
 9. A process according to claim 8 in which the anode is a ferro-silicon containing from 60 to 99 percent by weight of silicon.
 10. A process according to claim 9 where the process is conducted at substantially the boiling point of the liquid medium.
 11. A process according to claim 9 in which the liquid medium is a solution having a specific conductivity of at least 3.0 X 10 3 ohm1, cm 1 to about 3.0 X 10 5 ohm 1, cm 1 at the operating temperature. 